I'd like to add my success story.
My problem that lead me here was really, really poor Bluetooth, along with bad GPS (off by roughly 0.1 miles). GPS was useless for map directions and exercise apps.
I used app "GPS Status" by PocketGPSWorld.com to compare to other iPhones and mine showed "Poor Fix" with a horizontal accuracy of 65M. Speed and Course were listed as "Invalid". I presume this meant it would suck for direction/course based apps. Other iPhones showed significantly better info.
I bought the F-Type antenna for $4.90 here. Note: this item doesn't come with the tools for disassembly, so if you don't already have a kit, you'll have to buy that too.
I got the part and proceeded to disassemble my phone according to these instructions. When I opened my iPhone, the "sticker" that has been mentioned in this thread, which looks and tests like it's a piece of conductive foam, was a little scrunched up, moved and sticking to the underside of the metal camera housing. It certainly looked very out of place. Mine was a little too scrunched up to save, so I continued with the full replacement of the F-Antenna.
Disassembly tips: Use a piece of paper and tape to ensure you don't lose screws, and you can tape screws to the paper in the orientation the were removed. There are many different thread/length screws inside, so don't assume you can put them in a pile and remember where they go.
The replacement was successful and a retest of my GPS showed a status of an "Excellent Fix", a horizontal accuracy of 5M and Speed and Course showing data. I think it's very likely that re-placing the foam, if intact, will solve many problems and is way easier than a full disassembly. It would only require opening the phone, no other part removal would be required. However, my thought here is that if it came loose once, it may come loose again. If you're up to the task, I'd say replace the antenna (which includes a newly stuck on foam) and you'll be better off for the future.
Thanks a million to this thread!